The present invention generally relates to hard handoff in a multi-frequency CDMA network and particularly relates to dynamic generation of hard handoff targets.
Mobile communication networks, e.g., cellular networks, generally comprise a plurality of defined service areas referred to as cells. In networks based on Code Division Multiple Access (CDMA) techniques, each cell uses at least one CDMA channel, which represents the collection of forward and reverse radio links supporting communication between mobile stations in that cell and the network at a given carrier frequency. In this sense, each CDMA channel may be regarded as the intersection of a particular carrier frequency and a particular service area or cell. Where multiple carrier frequencies are used within one cell, that cell offers a like multiple of CDMA channels, each one operating in a different carrier frequency. More particularly, it should be understood that a typical CDMA channel is based on a defined forward link and reverse link frequency pair.
When a mobile station is admitted for service, it is assigned radio link resources on at least one CDMA channel at a particular carrier frequency, e.g., the mobile station is assigned to a particular forward/reverse link frequency pair. If that mobile station moves among cells while its connection is active, the network assigns needed radio link resources on the CDMA channel(s) associated with its new service location. The practice of transitioning radio service from one CDMA channel to another is referred to as “hand off.”
CDMA networks offer an opportunity to improve service reliability during handoff by exploiting “soft handoff” wherein the mobile station is simultaneously served by two or more CDMA channels. Soft handoff is referred to as a make-before-break handoff because, from the mobile station's perspective, forward link service begins on the new CDMA channel(s) before it ends on the mobile station's current CDMA channel(s). Indeed, the mobile station's geographic location may be such that it is served simultaneously by three or more CDMA channels. Of course, the benefits of soft handoff must be balanced against the consumption of system resources, since the mobile station must be allocated radio link resources on each CDMA channel used to serve it in soft handoff.
Soft handoff is not always desirable, or even possible. For example, the mobile station may move to a new cell that does not offer a CDMA channel on its current carrier frequency. As an example, assume that the mobile station is in Cell A operating on a CDMA channel in frequency F2, with this channel denoted as F2A, designating the CDMA channel corresponding to F2 in Cell A. Further, assume that the mobile station begins moving toward Cell B, which operates only with carrier frequency F1, i.e., the only CDMA channel available in Cell B is F1B.
Since the mobile station operates only on one carrier frequency at a time, it cannot be served by both F2A and F1B, and thus must undergo a “hard handoff” rather than a soft handoff. Hard handoffs generally are break-before-make handoffs arising from, as in the above example, a hard frequency handoff. Hard handoffs also are required where the control of a call is transitioning from one Base Station System (BSS) to another BSS. Indeed, the types of hard handoff include intra-BSS and inter-BSS, with the latter case being either an intra-MSC (same MSC) or inter-MSC (different MSCs) type of hard handoff.
Within these broad hard handoff types, there are two typical hard handoff triggering mechanisms. A first mechanism is based on Round-Trip-Delay (RTD), which is a measure of round-trip signal delay between a particular mobile station and a particular RBS. Since this delay time is directly proportional to distance, the RTD value may be used to sense when the mobile has moved so far from the serving RBS in a current cell that handoff to another cell is appropriate.
A second common triggering mechanism for hard handoffs involves Pilot Beacon Units (PBUs), which are special pilot signal transmitters that identify special border cells marking the transition between systems. As mobile stations report observed pilot signal strengths, the Base Station Controller (BSC) in the mobile station's current BSS can recognize the presence of a PBU channel pilot within a Pilot Strength Measurement Message (PSMM) from the mobile station. Thus, the current BSC recognizes when the mobile station is moving toward cells associated with another system by the presence of one or more PBU pilots in pilot signal reports from the mobile station.
Regardless of the triggering mechanism, hard handoffs in a conventional network rely on the use of pre-configured handoff targets. With this approach, each cell in the network has defined handoff targets that are specified, for example, as part of network provisioning operation. Significant maintenance overhead thus arises as the handoff target information must be maintained, updated, and its consistency verified, as the network changes, such as when cells are added and/or modified.
Among these configuration choices, system operators would have to pre-configure the type of hard handoff to be performed for a given CDMA channel (inter-BSS, intra-BSS), and would have to explicitly configure all of the potentially available target frequencies and cells available. Because of the static nature of these hard handoff configurations, such networks lose the advantage gained through dynamic knowledge of current radio frequency (RF) conditions, CDMA channel availability, etc. As a consequence, statically configured hard handoff types and targets often are too optimistic, leading to failed handoff and interrupted service, or are too pessimistic, leading to inefficient usage of network resources.
Thus, what is needed is a hard handoff method that dynamically determines one or more targets for the hard handoff of a mobile station. Preferably, this dynamic approach considers radio conditions and resource availability, and avoids the need for extensive, statically configured hard handoff target information.